When the Selegiline preparation is given together with Levodopa, it seems to increase and prolong the response time to Levodopa, and thus can reduce the erosion fluctuations (which were described in the Levodopa side effects section). Animal studies have shown that selegiline protects the dopamine-producing neurons from the toxic effect of MPTP. Selegiline inhibits or prevents the action of the MAO-B enzyme, which changes dopamine in the brain's metabolic process, and inhibits the breakdown of dopamine. The natural and the dopamine created from the levodopa. Dopamine is thus stored in the surviving neurons in the brain.

Some, but not all, doctors prefer to start selegiline in all Parkinson's patients because of the drug's possible protective effect. Selegiline is an easy drug to take, though that the side effects Its side effects may include nausea, orthostatic hypotension, or insomnia (when taken late at night). Toxic reactions also occurred in patients who took Selegiline with Fluoxetine (an antidepressant) and with Meperidine (a sedative and pain reliever).

Researchers are still trying to find answers to questions about the use of selegiline, such as how long the drug remains effective and whether there are negative effects with long-term use. Evaluating the long-term effects will help determine the value of the drug for all stages of the disease.